105 



s, gradually modifying the temporal lire of each inter- 

 mediate plane within tlie wall until a state of equilibrium is 

 established. Then heat will continue to tlow but the tem- 

 perature at each intermediate plane will stay unchanged. 

 Before the establishment of this equilibrium the system is 

 in a variable state; after the equilibrium is established the 

 system is in a steady state. In the variable state, the 

 amount of heat which enters through a given area on S is 

 different from the amount which comes out through an 

 equal area on s ; in the steady state these amounts are the 

 same. From theoretical considerations it has been con- 

 cluded that, in the steady state, the quantity of heat Q which 

 traverses the wall should be proportional to the difference 

 of temperature T-t, to the area A through which heat flows, 

 to the time z during which it flows and inversely propor- 

 tional to the thickness d of the wall : 



Q = K^^ (1) 



Experimental studies have confirmed this relation. 



The constant of proportionality K in the formula (1) is 

 the coefficient of heat conductivity. It is defined as the 

 number of calories which, during one second, traverse an 

 area of 1 cm" of a wall 1 cm. thick, when the temperatures 

 of the two sides of the wall difi'er by one degree. 



After the steady state is reached, each intermediate plane 

 p is at a temperature which is proportional to the distance 

 X from the plane p to the cooler surface (t being taken as 

 the temperature origin, that is, t = 0) : 



9 = ax (2) 



The constant of proportionality a is the ratio of the 

 temperature difference T-t to the thickness of the wall d. 



The formula (2) which is a necessary consequence of the 

 principles admitted in formula (1) has also been verified 

 experimentally. 



The main assumptions in these relations are: 1. That 

 the temperature on each side of the wall is constant; 2. 



